The overall objective of this study is to contribute to a better understanding of the basic mechanisms involved in mammalian fertiliation. Using improved in vitro capacitation systems that clearly separate capacitation from the acrosome reaction, we will study how various cations, energy sources, fatty acids, retinol and related compounds and inhibitors of cytoskeletal systems affect capacitation and the acrosome reaction. Little is known about capacitation and the acrosome reaction of spermatozoa in vivo. We will study whether spermatozoa in the isthmic region of the oviduct are fully capacitated before they ascend to the ampulla where fertilization takes place. We will also determine whether the cumulus cophorus has the ability to induce the acrosome reaction. Relatively little attention has been paid to hyperactivation that spermatozoa of many species display about the time of fertilization. Mechanisms underlying this phenomenon unique to mammals will be investigated using both membrane-intact and demembranated spermatoaoa. A chitin-binding substance we have recently found in spermatozoa is a possible candidate for the substance(s) that is utilized by spermatozoa to recognize the zona pellucida and the egg plasma membrane. Its subcellular localization and involvement in sperm-egg interactions will be determined. The sperm plasma membrane becomes capable of fusing with the egg plasma membrane only after the acrosome reaction. We will study what sorts of membrane changes occur concomitant with the acrosome reaction. We will also determine when the egg plasma membrane gains the ability to fuse with the sperm plasma membrane. After sperm-egg fusion, the highly condensed sperm nucleus must decondense and sperm and egg pronuclei must migrate to the center of the egg. We will study the origin of the sperm nucleus-decondensing factor and the influence of cytoskeletal system inhibitors on the development and migration of pronuclei.